Radio frequency identification (RFID) generally employs resonance in order to increase the efficiency of energy transfer from the reader to the tag. This is achieved through the resonant recycling of energy that results in voltage step up in the tag when subject to the reader powering field. In order to achieve the required minimum voltage for the tag to operate, a high Q resonance in the tag is desirable. However, two constraints mean that the tag Q is limited to low values of order 10:    1) As the tag Q increases the width of the resonance drops proportionally. The high levels of step up corresponding to the high Q are only achieved over a very narrow frequency band. This makes the system susceptible to severe degradation due to environmental detuning or variation at manufacture.    2) The system is also required to communicate once the tag is powered. A high tag Q does not give sufficient bandwidth for communication and the amplitude of any load modulation, when registered at the reader, is attenuated.
Some of these limitations are addressed in PCT/GB2006/050436 where a new resonant circuit is disclosed comprising a self-adaptive resonator that may be operated over a designed band of frequencies, independent of the level of loss. In embodiments this is achieved through the use of an antenna and two capacitive paths that are coupled into the resonance with a variable duty cycle; the duty cycle is controlled by the waveform amplitude and the gate voltage on a MOSFET. One application of this circuit is in a tag where the induced voltage is used to control the mosfet gate voltage and ramp up the amplitude in tag. This arrangement can achieve high levels of voltage step up corresponding to low loss in the antenna resonance, without the drawbacks of a narrow single resonance frequency. In effect the tag has an auto tuning behaviour to the stimulus frequency, provided it is within a designed frequency band. Furthermore, the system may be completely passive without the requirement for a separate power source to operate the tuning circuit. It therefore has application in RFID with potential benefits including:    1) Extending the range over which the tag may be powered.    2) Allowing the use of a smaller tag antenna for the same range    3) Making the tag more tolerant to environmental detuning or variation at manufacture.
Once the tag is powered it is then required to communicate with the reader through load modulation. A method is disclosed in PCT/GB2006/050440 whereby feedback is employed in a reader to reduce the variation in resonance amplitude in response to tag load modulation. This has the effect of increasing the speed of response in the reader to variation on the modulation timescale, while retaining the low loss behaviour of the slowly varying powering field. The result is a low power reader, also capable of achieving excellent range.
To date there is no disclosure outlining how to achieve this same separation of dynamics in a tag whereby high Q operation is achieved for a slowly varying powering field together with a fast response on the timescale of the communication modulation. This is needed in order to carry out the required functions of tag reading and tag programming.